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<a name="RTL-SSA-Instructions"></a>
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Next: <a href="RTL-SSA-Basic-Blocks.html#RTL-SSA-Basic-Blocks" accesskey="n" rel="next">RTL SSA Basic Blocks</a>, Previous: <a href="Using-RTL-SSA.html#Using-RTL-SSA" accesskey="p" rel="prev">Using RTL SSA</a>, Up: <a href="RTL-SSA.html#RTL-SSA" accesskey="u" rel="up">RTL SSA</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="RTL-SSA-Instructions-1"></a>
<h4 class="subsection">14.21.2 RTL SSA Instructions</h4>

<a name="index-RPO"></a>
<a name="index-reverse-postorder"></a>
<a name="index-instructions_002c-RTL-SSA"></a>
<a name="index-rtl_005fssa_003a_003ainsn_005finfo"></a>
<p>RTL SSA instructions are represented by an <code>rtl_ssa::insn_info</code>.
These instructions are chained together in a single list that follows
a reverse postorder (RPO) traversal of the function.  This means that
if any path through the function can execute an instruction <var>I1</var>
and then later execute an instruction <var>I2</var> for the first time,
<var>I1</var> appears before <var>I2</var> in the list<a name="DOCF4" href="#FOOT4"><sup>4</sup></a>.
</p>
<p>Two RTL SSA instructions can be compared to find which instruction
occurs earlier than the other in the RPO.  One way to do this is
to use the C++ comparison operators, such as:
</p>
<div class="example">
<pre class="example">*<var>insn1</var> &lt; *<var>insn2</var>
</pre></div>

<p>Another way is to use the <code>compare_with</code> function:
</p>
<div class="example">
<pre class="example"><var>insn1</var>-&gt;compare_with (<var>insn2</var>)
</pre></div>

<p>This expression is greater than zero if <var>insn1</var> comes after <var>insn2</var>
in the RPO, less than zero if <var>insn1</var> comes before <var>insn2</var> in the
RPO, or zero if <var>insn1</var> and <var>insn2</var> are the same.  This order is
maintained even if instructions are added to the function or moved around.
</p>
<p>The main purpose of <code>rtl_ssa::insn_info</code> is to hold
SSA information about an instruction.  However, it also caches
certain properties of the instruction, such as whether it is an
inline assembly instruction, whether it has volatile accesses, and so on.
</p>
<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT4" href="#DOCF4">(4)</a></h3>
<p>Note that this
order is different from the order of the underlying RTL instructions,
which follow machine code order instead.</p>
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